Telescopes are used to collect and analyze radio waves, microwaves, light, and other radiation, astronomers have devised a variety of telescopes, which are devices that focus and concentrate radiation from distant objects.
The first telescopes could examine only visible light, and when most people use the word “telescope” today, they mean an instrument for gathering and concentrating this form of radiation. The classic reflecting telescope has a large mirror that reflects and focus light to produce an image of the object being studied.
Many modern light-gathering telescopes are built differently. Instead of having a solid block of glass for a mirror, they have an array of small, independently controlled, light-weight mirrors that, taken together, produce an image. The Keck telescope on Mauna Kea in Hawaii, the biggest of these new-age instruments with a collection of more than a dozen mirrors, together forming a reflective surface about 10 meters (30 feet) in diameter, was put into operation in 1992. Other factors being equal, the larger the telescope, the more useful it will be, because it will be able to collect more photons and thus detect fainter objects.
In the 1930s, astronomers built radio receivers that did for radio waves what the reflecting telescope did for light waves. For the first time, they could look at another kind of electromagnetic radiation. Today, large radio telescope facilities can be all around the world.